The present invention relates to a new coating composition based on an oxazolidine functional compound, an isocyanate functional compound, and a compound selected from a mercapto and a sulfonic acid functional compound.
Coatings used for painting motor vehicles and repairing the original paint are required to have good physical properties such as hardness, mechanical strength, and resistance to water, acids, and solvents. The coatings are also required to have good appearance properties, which means that films must be smooth and have a high gloss and high distinctness of image (DOI). It is also desirable that all properties be retained under prolonged outdoor weathering.
For environmental reasons, it is required to use a coating composition which can be applied easily using spray application at a low volatile organic content (VOC). Coatings with a lower organic solvent content emit lower levels of solvent when they are used and so the atmosphere becomes less polluted.
One way to achieve a lower solvent content is to use so-called high-solids compositions. Such compositions comprise a relatively high level of non-volatile materials, such as film forming polymer, pigments and fillers, and a relatively low level of organic solvent. A problem when formulating high-solids coating compositions is that such compositions have an unacceptably high viscosity due to the high molecular weight of the conventional film forming polymer. The high viscosity gives rise to problems in spray application with poor paint atomization and poor flow-out and, consequently, low gloss levels and poor appearance.
The use of low-molecular weight film forming polymers, which results in adequate application viscosities, has as a disadvantage that the resulting coating is soft and marks easily. The hardness build-up of the coating is therefore unacceptable.
Another way to reduce the amount of volatile organic compounds in coating compositions is the use of reactive diluents. Examples of reactive diluents include aldimines and ketimines. In EP-A-0 686 654 such compounds are discussed. Other reactive diluents, such as oxazolidines, are mentioned but it is stated that they have limited utility as a sole reaction partner with isocyanate because of generally slow setting film properties.
Aldimines are used as reactive diluents for low VOC paint systems. These diluents are commercially available and use of such aldimines is referred to in U.S. Pat. No. 5,214,086 and EP-A-0 686 654. In general these reactive diluents offer good cure and hardness development. However, they are known to cause skin irritation and to cause adhesion failure when applied in low VOC clearcoats.
Bicyclic oxazolidines are also used as reactive diluents for low VOC paint systems. These diluents are commercially available and use of such bicyclic oxazolidines are referred to in WO 95/14528. In general, these reactive diluents offer good durability and color stability and low toxicity. However, compositions based on bicyclic oxazolidines may exhibit an unacceptably long time to cure.
Monocyclic oxazolidines are also used as reactive diluents for low VOC paint systems. Coating compositions based on monocyclic oxazolidines are also referred to in EP 0 499 188 A1. Monocyclic oxazolidines and dimers of the same are commercially available. Coating compositions based on monocyclic oxazolidines may exhibit a loss of adhesion over time.
While the use of mono- or bi-cyclic oxazolidines has been described (see, e.g., U.S. Pat. No. 5,126,421), nowhere is there described the use of both monocyclic and bicyclic oxazolidines. WO 92/13907 refers to the use of monocyclic and bicyclic oxazolidines (page 7, lines 20-21), but this reference relates to the use of these compounds in the alternative and does not disclose a mixture of the two oxazolidines.
U.S. Pat. No. 5,506,328 refers to a two-component system with side A being an aliphatic polyisocyanate and side B being a tertiary amine catalyst and optionally an isocyanate-reactive compound selected from the group consisting of monoahls, polyols, imines, oxazolidines, or combinations thereof. The monoahl can be an -SH compound. The examples show only alcohols as monoahls. It has not been recognized by this disclosure that the combination of an oxazolidine functional compound, an isocyanate functional compound, and a mercapto functional compound shows improved and unexpected results over the use of the combination of an oxazolidine functional compound and an isocyanate functional compound alone.
Surprisingly, it was found that the addition of a mercapto functional compound to a coating composition comprising an oxazolidine functional compound and an isocyanate functional compound results in an increased potlife while maintaining the drying characteristics of the composition or results in increased drying rate while maintaining acceptable potlife.
It was also found that the addition of a sulfonic acid functional compound to a coating composition comprising an oxazolidine functional compound and an isocyanate functional compound results in improved curing characteristics of the coating composition, even at low relative humidity (e.g., 30% and below), without loss of potlife and, in some cases, with increased potlife.
It was also found that the use of mixtures of bicyclic and monocyclic oxazolidines as reactive diluents in low VOC coatings provide properties that can compete with those of imines, but without loss of adhesion.
A lower VOC at the same viscosity and a higher pot life/drying balance is obtained when using coating compositions of the present invention while other paint properties remain on the same level.
The present invention relates to a coating composition comprising:
(a) at least one oxazolidine functional compound,
(b) at least one isocyanate functional compound, and
(c) at least one compound selected from the group consisting of a mercapto functional compound, a sulfonic acid functional compound, and mixtures thereof.
Component (a) comprises at least one oxazolidine functional compound which may be a monocyclic oxazolidine functional compound, a bicyclic oxazolidine functional compound, and, preferably, mixtures thereof.
Component (b) comprises at least one isocyanate functional compound which may be an isocyanurate, a uretdione, a biuret, an allophonate, an adduct, a NCO prepolymer, or mixtures thereof.
Component (c) may be a mercapto functional compound, preferably a mercapto functional silane, most preferably xcex3-mercapto-propyl-trimethoxysilane. Component (c) may also be a sulfonic acid functional compound, preferable p-toluene sulfonic acid or dodecyl benzene sulfonic acid. Component (c) more preferably comprises a mixture of mercapto and sulfonic acid functional compounds, most preferably a mixture of xcex3-mercapto-propyl-trimethoxysilane and p-toluene sulfonic acid.
The composition may also further comprise:
(d) a resin comprising a functional group selected from hydroxyl and amine.
The invention further relates to a method of coating a substrate with the coating composition and to a substrate coated with the coating composition.
Component (a) comprises at least one oxazolidine functional compound. Preferably, the oxazolidine functional compound has the formula: 
wherein
x and y are independently selected from 0 to 10, with the proviso that x and y cannot both be 0;
n is selected from the integers 2 or 3;
m and p are independently selected from the integers 1 or 2;
R1, R2, R3, R4, R5 and R6 may be the same or different and are selected from the group of hydrogen, linear or branched (cyclo)alkyl, and linear or branched aryl, optionally substituted;
R1 and R2, R3 and R4, and R5 and R6 may be joined together to form a 5 or 6 carbon ring with the attached carbon atom of the ring in the formula, i.e. R1 and R2, R3 and R4, and R5 and R6 collectively represent a tetramethylene or a pentamethylene group; and
R7 is a mono- or multivalent aliphatic, aromatic, arylaliphatic or cycloaliphatic moiety which may optionally contain oxygen, nitrogen, sulphur, and silica, and R7 may be H when x or y is zero.
The variables recited in the formula are intended to be chosen independently both within one moiety, from one moiety to the next, and from one compound to the next.
More preferably, the oxazolidine functional compound is selected from one of the following formulae II (monocyclic oxazolidine functional compounds) and III (bicyclic oxazolidine functional compounds). 
wherein n, m, p, R1, R2, R3, R4, R5, R6, and R7 are as mentioned above and z is from 0 to 9.
Preferably, n is 2 and m and p are 1. Preferably, z is from 0 to 3.
Preferably, R1, R2, R3, R4, R5, and R6 are selected from the group of hydrogen, phenyl, benzyl or a linear or branched C1-12 alkyl group. More preferably, R1, R2, R3, R4, R5, and R6 are selected from the group of hydrogen and isopropyl.
In formula II, preferably, R7 is multivalent and more preferably R7 is an aliphatic, arylaliphatic or cycloaliphatic moiety comprising 2 to 15 carbon atoms and, optionally, ester, carbonate, and urethane groups. Monocyclic oxazolidine functional compounds wherein R7 comprises ester groups may be based on acrylate polymers such as described in GB-B-992,721.
More preferably, z is 1 and R7 is a divalent moiety comprising either a carbonate group or at least two urethane groups.
Compounds having an R7 group comprising carbonate groups are described in EP-A-0 499 188. R7 may be for example 
When R7 comprises at least two urethane groups, R7 may be selected from the group of the following multivalent moieties: 
Most preferably, z is 1, n is 2, R1 is hydrogen, R2 is isopropyl, and R7 is: 
This embodiment is commercially available under the tradename xe2x80x9cINCOZOL(trademark)xe2x80x9d LV (from Industrial Copolymers Ltd., Preston, Lancashire, UK).
Component (a) may comprise a mixture of monocyclic oxazolidine functional compounds according to formula II.
In formula III, preferably, z is 0 and R7 is an alkyl group comprising 1 to 10 carbon atoms, more preferably methyl, ethyl or propyl. Most preferably, the oxazolidine functional compound comprises 1-aza-3,7-dioxo-2,8-diisopropyl-5-ethyl bicyclo(3,3,0)octane, which is available commercially under the tradename xe2x80x9cZOLDINE(copyright)xe2x80x9d RD-20 from Angus Chemical Company (Buffalo Grove, Ill.) and has the structure: 
Component (a) may comprise a mixture of bicyclic oxazolidine functional compounds according to formula III.
Most preferably, component (a) comprises a mixture of a monocyclic oxazolidine functional compound of the formula II and a bicyclic oxazolidine functional compound of the formula III. When such a mixture is used, the weight ratio of a bicyclic oxazolidine functional compound of the formula III to a monocyclic oxazolidine functional compound of the formula II is preferably in the range of 5:1 to 1:5, more preferably 3:1 to 1:3, most preferably, 1:1 ratio.
Component (b) comprises at least one isocyanate functional compound. The isocyanate functional compound can be an aromatic, aliphatic, cycloaliphatic and/or araliphatic isocyanate functional compound. The isocyanate functional compound can be an isocyanurate, uretdione, biuret, allophanate, an adduct, NCO prepolymers, or mixtures thereof.
Examples of suitable isocyanates to be used as the isocyanate functional compound, or as starting materials for preparing an isocyanate functional compound comprising an isocyanurate, biuret or uretdione structure include organic polyisocyanates represented by the formula
R(NCO)k
wherein k is 2 or higher and R represents an organic group obtained by removing the isocyanate groups from an organic polyisocyanate having aromatically or (cyclo)aliphatically bound isocyanate groups. Preferred diisocyanates are those represented by the above formula wherein k is 2 and R represents a divalent aliphatic hydrocarbon group having 2 to 18 carbon atoms, a divalent cycloaliphatic hydrocarbon group having 5 to 15 carbon atoms, a divalent araliphatic hydrocarbon group having 7 to 15 carbon atoms or a divalent aromatic hydrocarbon group having 6 to 15 carbon atoms. Examples of the organic diisocyanates which are particularly suitable include ethylene diisocyanate, 1,3-propylene diisocyanate 1,4-tetramethylene diisocyanate, 1,6-hexamethylene diisocyanate, 2,2,4-trimethyl-1,6-hexamethylene diisocyanate, 2-methyl-1,5-diisocyanate pentane, 2-ethyl-1,4-diisocyanate butane, 1,12-dodecamethylene diisocyanate, cyclohexane-1,3- and -1,4-diisocyanate, 1-isocyanato-2-isocyanatomethyl cyclopentane, isophorone diisocyanate, bis-(4-isocyanatocyclohexyl)-methane, 2,4xe2x80x2-dicyclohexylmethane diisocyanate, 1,3- and 1,4-bis(isocyanatomethyl)-cyclohexane, bis-(4-isocyanato-3-methyl-cyclohexyl)-methane, 1-methyl-2,4-diisocyanato cyclohexane, 1-isocyanato-1-methyl-4(3)-isocyanatomethyl cyclohexane, xylene diisocyanate, 1-methyl-2,4-diisocyanato benzene, xcex1,xcex1,xcex1xe2x80x2,xcex1xe2x80x2-tetramethyl-1,3- and -1,4-xylylene diisocyanate, 2,4- and 2,6-hexahydrotoluylene diisocyanate, 1,3- and 1,4-phenylene diisocyanate, 2,4- and 2,6-toluylene diisocyanate, 2,4- and 4,4xe2x80x2-diphenylmethane diisocyanate, 1,5-diisocyanato naphthalene and mixtures thereof. Aliphatic polyisocyanates containing 3 or more isocyanate groups such as 4-isocyanatomethyl-1,8-octane diisocyanate and aromatic polyisocyanate containing 3 or more isocyanate groups such as 4,4xe2x80x2,4xe2x80x3-triphenylmethane triisocyanate, 1,3,5-triisocyanate benzene, polyphenyl polymethylene polyisocyanates obtained by phosgenating aniline/formaldehyde condensates, and mixtures thereof may also be used.
Isocyanate functional compound comprising an allophanate structure are prepared by the reaction of the above-mentioned organic polyisocyanates with a mono- or polyalcohol. Preferably, isocyanate functional compound comprising an allophanate structure are prepared from 1,6-hexamethylene diisocyanate and/or isophorone diisocyanate reacted with an alcohol, preferably butanol. Other allophanate structures are disclosed in copending application having U.S. Ser. No. 08/906,644, filed Aug. 7, 1997, which is hereby incorporated by reference.
Polyisocyanate adducts include the adduct of trimethylol propane and m-tetramethylxylylene diisocyanate and the adduct of trimethylol propane and toluene diisocyanate.
The NCO prepolymers are prepared from the previously described monomeric polyisocyanates, preferably monomeric diisocyanates, and organic compounds containing at least two isocyanate-reactive groups, preferably at least two hydroxy groups. These organic compounds include high molecular weight compounds having number average molecular weights of 400 to about 6,000, preferably 800 to about 3,000, and optionally low molecular weight compounds with molecular weights below 400. Products obtained by reacting polyisocyanates exclusively with low molecular weight compounds are polyisocyanate adducts containing urethane groups and are not considered to be NCO prepolymers.
Preferably, component (b) comprises diisocyanates selected from 1,6-hexamethylene diisocyanate (HDI), isophorone diisocyanate (IPDI), and toluene diisocyanate (TDI). More preferably, component (b) comprises the isocyanurate and uretdione structures of HDI, IPDI, and TDI. Most preferably, component (b) is the isocyanurate of HDI.
Component (c) comprises at least one compound selected from the group consisting of a mercapto functional compound, a sulfonic acid functional compound, and mixtures thereof.
The mercapto functional compound is preferably a compound having primary mercapto groups. Examples include pentaerythritol tetra(3-mercaptopropionate), pentaerythritol tetra(thioglycolate), trimethylol propane tri(3-mercaptopropionate), trimethylol propane tri(thioglycolate), and mercapto functional silane compounds, such as xcex3-mercapto-propyl-trimethoxysilane, xcex3-mercapto-propyl-triethoxysilane, xcex3-mercapto-propyl-methyldimethoxysilane, xcex2-mercapto-ethyl-trimethoxysilane, and xcex2-mercapto-ethyl-triethoxysilane. Most preferably, xcex3-mercapto-propyl-trimethoxysilane, available from Hxc3xcls under the tradename xe2x80x9cDYNASYLAN MTMOxe2x80x9d is used.
The mercapto functional compound is preferably present in an amount of about 0.1 to about 2.0 wt. % of a ready-to-spray coating composition, more preferably about 0.2 to about 0.6 wt. %, most preferably about 0.2 to about 0.4 wt. %. The optimum amount is dependent on the amount of catalyst.
The use of the mercapto functional compound has been shown to increase potlife while maintaining similar drying characteristics or of increasing the drying rate while maintaining acceptable potlife.
While not wishing to be bound by theory, it is believed that these unexpectedly superior results are related to the complexing nature of sulfur atoms. The mercapto group is thought to complex the commonly used tin catalyst in the cup, whereas surprisingly the curing is not adversely affected.
The sulfonic acid functional compound can be a strong acid such as p-toluene sulfonic acid or dodecyl benzene sulfonic acid. The amount added depends on the sulfonic acid functional compound used. The sulfonic acid functional compound can be present in an amount of about 0.2 to 2.0 wt. %, more preferably 0.3 to 1.6 wt. %, most preferably 0.6 to 1.0 wt. % of a ready-to-spray coating composition.
The addition of the sulfonic acid functional compound surprisingly imparts improved curing characteristics to the coating composition. It can be cured without leaving a tacky film, even at low relative humidity (at or below 30%). There is no adverse effect on the potlife, and, in fact, the potlife may show an increase in some cases.
While not wishing to be bound by theory, it is believed that the sulfonic acid functional compound breaks open the oxazolidine structures, and it is surprising that this does not reduce potlife. Other acids such as organic and metallic acids were tested but none were found to provide the same benefits as sulfonic acid functional compounds.
Component (d) comprises a resin comprising a functional group selected from hydroxyl and amine. The hydroxyl functional resin may be an acrylic, polyester, polyether, polyurethane functional resin or other constituent materials known in the art, and may be a mixture thereof. Examples include hydroxyl-functional binders, e.g., polyester polyols such as described in H. Wagner et al., Lackkunstharze, 5th ed., 1971 (Carl Hanser Verlag, Munich), polyether polyols, polyacrylate polyols, polyurethane polyols, cellulose acetobutyrate, hydroxyl-functional epoxy resins, alkyds, ketone resins, and dendrimeric polyols such as described in WO 93/17060 Also, hydroxyl-functional oligomers and monomers, such as castor oil and trimethylolpropane may be present.
Alternately, component (d) comprises an amine functional resin. Suitable compounds may be aliphatic, aromatic, cycloaliphatic and/or araliphatic, may contain a saturated, an unsaturated group, O, S or N, and include ethylene diamine, ethylene glycol diamine, propylene glycol diamine, and cycloaliphatic diamines. Examples include aspargyl acid esters, and latent or non-latent amino-functional compounds such as ketimines, aldimines, diimines, secondary amines, and polyamines may be present. These and other compounds are known to the skilled person and are mentioned, int. al., in U.S. Pat. No. 5,214,086. Preferably, the amine functional resin is an aspartic acid ester. Suitable resins are available commercially from Bayer under the tradenames xe2x80x9cDesmophen(copyright)xe2x80x9d XP 7052 (sterically hindered aminesxe2x80x94adduct of 2 moles diethylmaleate with the amine xe2x80x9cLaromin(copyright)xe2x80x9d C 260 (BASF, Germany)) and xe2x80x9cDesmophene(copyright)xe2x80x9d XP 7053 (adduct of 2 moles diethylmaleate with the amine PACM 20).
Preferably, component (d) is a hydroxyl functional resin.
Preferably, the percentages by weight of components (a) to (d) on total vehicle solids are about: (a) 5-60 wt. %, (b) 30-70 wt. %, (c) 0.3-4.0 wt. %, and (d) 0-30 wt. %.
The coating composition of the present invention may also comprise hydroxyl or other functional reactive diluents, which may lower the viscosity, lower the VOC and boost the reactivity of the coating composition.
Preferably, the ratio of isocyanate groups to isocyanate-reactive groups is 0.25 to 1.5, preferably 0.75 to 1.25.
The coating composition of the present invention may also comprise additional components such as solvents, catalysts, stabilizers, fillers, rheology control agents, flow additives, leveling additives, dispersing agents and other components known to persons skilled in the art. Suitable solvents include methyl amyl ketone, butyl acetate, amyl acetate, ethoxy ethyl propionate and xylene. Suitable catalysts include aromatic or aliphatic carboxylic acids, and organometal compounds. Acids which are useful are formic acid, acetic acid, mono-, di-, and trichloro acetic acid, oxalic acid, maleic acid, malonic acid, fumaric acid, heptanoic acid, pelargonic acid, isononanoic acid, benzoic acid, 4-hydroxybenzoic acid, mono-, di-, and trichlorobenzoic acid, and salicylic acid and anhydrides thereof. Preferred acids are acetic acid, heptanoic acid, and benzoic acid. Useful organometal compounds include zinc alkanoate, such as zinc octoate, dibutyltin dilaurate, dibutyltin (bis)mercaptide, dibutyltin diacetate, and dibutyltin sulphide. Preferred is an organo-tin catalyst such as dibutyltin dilaurate. Also mixtures of the above-mentioned catalysts may be used.
Optionally pigments may be present in the coating composition of the present invention. Useful pigments are various types common to the art which include but are not limited to titanium dioxide, graphite, carbon black, zinc oxide, calcium sulphide, chromium oxide, zinc sulphide, zinc chromate, strontium chromate, barium chromate, lead chromate, lead cyanamide, lead silico chromate, yellow nickel titanium, yellow chromium titanium, red iron oxide, yellow iron oxide, black iron oxide, naphtol red and browns, anthraquinones, dioxa zinc violet, isoindoline yellow, arylide yellow and oranges, ultramarine blue, phthalocyanine complexes, amaranth, quinacridones, halogenated thioindigo pigments, extender pigments such as magnesium silicate, aluminum silicate, calcium silicate, calcium carbonate, fumed silica, barium sulfate, and zinc phosphate.
Preferably, the coating composition comprises less than 500 g/l (4.2 lbs/gal) of volatile organic solvent based on the total composition, more preferably less than 480 g/l (4.0 lbs/gal), most preferably less than 420 g/l (3.5 lbs/gal). The solid resin content preferably is higher than 50%, more preferably higher than 52%, most preferably higher than 58%.
Coating compositions of the present invention are useful as clearcoats, basecoats, topcoats, and primers.
The coating composition of the present invention can be used in the preparation of coated substrates. These substrates include glass, ceramics, paper, wood, plastic and metal.
The coating compositions may be formulated in a 2-, 3-, or 4-component system, depending on the choice of the components (a), (b), (c), and, optionally, (d). For example, the sulfonic acid functional compound may be added as the fourth component to a 3-component system consisting of component (a), (b), and (d) or it can be added as part of one of the existing components, such as component (d).
The coating composition is especially useful in the refinish industry, in particular the body shop, to repair automobiles. The coating composition is also applicable in the automotive industry for the finishing of large transport vehicles, such as trains and buses, and can also be used in airplanes. The substrate may be uncoated material or can be primed. The substrate may also be coated with paint products applied at the time of manufacture or just prior to application of the compositions of the present invention. The coating composition can be applied using conventional spray equipment or high volume low-pressure spray equipment resulting in a high quality finish. Other modes of application are roller coating, brushing, sprinkling, flow coating, dipping, electrostatic spraying or electrophoresis, with spraying being preferred. Exemplary metal substrates include steel, aluminum, copper, zinc, magnesium, and alloys thereof. Curing temperatures are preferably between 0 and 80xc2x0 C., and more preferably between 20 and 60xc2x0 C.